1. Field of the Invention
This invention relates to gaseous discharge lamps which ignite at voltages that are much higher than their operating voltages and, in particular, to the igniting of such lamps.
2. Description of Related Art
Common characteristics of a gaseous discharge lamp are its negative resistance and high igniting voltage. Circuitry for powering such a lamp typically includes a current limiting means, such as a ballast, to compensate for the negative resistance, and often includes igniter circuitry for generating high-voltage pulses to ignite the lamps. Such igniter circuitry commonly includes a voltage-sensitive switch (e.g. a sidac) for effecting the continual production of the high-voltage pulses until the lamp ignites. Upon ignition, the voltage across the lamp decreases from a higher open-circuit voltage (OCV) to a lower voltage, which causes the switch to change to a nonconducting state and to effect termination of pulse production. One example of such a ballast is described in U.S. Pat. No. 5,825,139.
Typically, a cable separates the igniter circuit from the gaseous discharge lamp. The distance between gaseous discharge lamp and the ballast is typically referred to the xe2x80x9cBTLxe2x80x9d distance, i.e. the ballast-to-lamp distance. As the BTL distance increases, the loading effect of parasitic capacitances associated with the cable increases thereby degrading the igniter voltage pulse actually delivered to the lamp. The parasitic loading capacitance is directly proportional to the BTL distance. Typically, for significant BTL distances, the igniter voltage pulse actually applied to the lamp does not even meet the minimum ANSI specifications. Thus, without any compensation, the peak voltage delivered to the lamp would tend to decrease with increases in parasitic loading capacitance.
One attempt to address the aforementioned problem is to configure the ignitor so as to increase its output voltage. As a result, when the BTL distance is relatively long, the resulting magnitude of the igniter voltage pulse applied to the lamp complies with ANSI specifications and is sufficient to properly illuminate the lamp. However, such a high-output igniter circuit is not suitable for use when the BTL distance is relatively short. In such a situation, the relatively high magnitude of the igniter voltage pulse is far greater than the maximum ANSI specifications. The application of such a high-magnitude ignitor pulse to the lamp usually reduces the life of the lamp and may even cause damage to the lamp and other ballast components.
What is needed is a single igniter circuit that can be used for both relatively short or long BTL distances.
In accordance with the invention, circuitry is provided for powering a gaseous discharge lamp that has a required minimum ignition voltage and a permissible maximum ignition voltage. The circuitry comprises an output configured for connection to a cable through which ignition voltage pulses are delivered to the lamp wherein the cable has a predetermined parasitic loading capacitance that depends upon the length of the cable and wherein the predetermined parasitic loading capacitance is within a range of parasitic loading capacitances defined by a predetermined minimum capacitance and a predetermined maximum capacitance. The circuitry further comprises a source of ignition pulses including an energy source capable of outputting an ignition voltage into a cable that has the predetermined maximum parasitic loading capacitance and charging that predetermined maximum parasitic loading capacitance to at least the required minimum ignition voltage of the lamp. The energy source comprises a first energy source and a second energy source. The source of ignition pulses has a first state of operation wherein the first energy source charges the parasitic loading capacitance of the cable to a voltage that does not exceed the permissible maximum voltage of the lamp when the parasitic loading capacitance of the cable is within a first predetermined range of capacitances. The source of ignition pulses has a second state of operation wherein the second energy source charges the parasitic loading capacitance of the cable to at least the required minimum ignition voltage of the lamp when the parasitic capacitance of the cable is within a second predetermined range of capacitances. Each capacitance of the second range is larger than every capacitance of the first range.
In one embodiment, the first and second energy sources each comprise a capacitive source.
A key feature of the present invention is that the reactive storage capacity of a ballast is varied to compensate for the particular parasitic loading capacitances associated with a cable that delivers the ignition voltage to the lamp. Thus, the peak voltage delivered to the lamp does not exceed the permissible maximum voltage when the cable is relatively short, and does not fall below a required minimum voltage when the cable is relatively long.
An advantage of the present invention is that such compensation for the parasitic loading capacitances of the cable are achieved without substantially increasing the cost or complexity of the ballast.